In engines, gas of a combustion chamber flows through a gap between an engine cylinder and a piston into a crankcase as blow-by gas. Many of the engines include a breather device for causing the blow-by gas to flow out of the crankcase and then return to the combustion chamber, as disclosed for example in Japanese Patent No. 4089334. According to the breather device disclosed in the 4089334 patent, once the inner pressure of the crankcase exceeds a predetermined pressure due to the blow-by gas, a breather valve (lead valve) of the breather device opens, so that the blow-by gas is sucked into an air intake system via a breather passage. The blow-by gas thus sucked into the air intake system is returned back to the combustion chamber of the engine.
For stability during transportation or storage of the engine, it is sometimes desired that the engine be retained in a laterally laid-down position, i.e. in a position where the engine cylinder lies sideways. However, the engine disclosed in the 4089334 patent is constructed for use in a vertically upright position (i.e., position where the engine cylinder is located in an upper region of the engine). Thus, when the engine is placed in the laterally laid-down position, lubricating oil stored in the crankcase may enter the breather passage. Consequently, if the engine is driven after (particularly, immediately after) the engine is returned from the laterally laid-down position to the vertically upright position, the lubricating oil staying in the breather passage may be undesirably sucked into an air intake system of the engine.